Condition monitoring equipment such as oxygen monitors for hospital use have long employed AC powered circuitry, micro fuel cells and meter indicators. More recently hospital monitoring equipment has been battery powered, where possible, to avoid the exposure of patients to an alternating current environment whenever possible. This has been important particularly in the oxygen therapy field where increased levels of oxygen can be associated with explosion and fire danger, and in the case of electrically sensitive patients, e.g. those with implanted electrodes, pace-makers and the like. Examples of battery powered medical equipment are disclosed in U.S. Pat. Nos. 3,785,207 and 3,765,244 to J. Brzezinski, and examples of battery powered oxygen monitoring equipment is disclosed in U.S. Pat. No. 3,375,700 to R. Hubner.
Typically, such equipment to provide accurate reading must regulate the power supply and such is accomplished in the Brzezinski patents referenced above employing zener diodes. I have discovered however that the current drain of the zener diode in providing voltage regulation is excessive and I have therefore proceeded to develop a non-zener diode regulation circuit which is simple and of low current drain.
It has been the practice in recent years in battery powered electronic equipment to provide a battery condition indicator. Such a condition indicator in oxygen monitoring equipment is shown in U.S. Pat. No. 3,375,700. The battery condition indication, however, is independent of the displayed data.
I have further discovered that the typical meter employed to register a condition such as oxygen concentration at best has an accuracy in the order of 1 to 2 percent in actual reading. Also, because of parallax and other problems of the sensitivity of meter reading, quite often the error in the actual reading recorded or noted may be easily 5%. Under such conditions, the accurate, precise monitoring of a condition such as oxygen concentration is impractical if not impossible.
In the past, medical equipment including alarm circuits when a parameter exceeds a limit, has been employed. I have discovered, however, that most of such alarm circuits are of an analog nature and and that the error inherent in or resulting in the limit circuit may be significantly greater than the sensing accuracy. In such case, the very condition which one seeks to eliminate may occur and the user has a false sense of security because of the monitoring and limit functions provided by the equipment.